Allow building just the sdk installers in dotnet/sdk

[ViktorHofer]

When building the sdk installers in BuildPass=2 on Windows (as they depend on join assets from different verticals) the redist-installers.csproj project needs to be referenced in the repo's Build.props file. This causes the entire archive to be built again which is throw-away work and results in duplicate assets when merged and uploading everything. This is especially important as that logic will run in a BuildPass leg which means that it's on the critical path from a build time perspective.

It should be possible to encapsulate the logic that is responsible for creating the installers in dotnet/sdk and only invoke that.